People say that you need a data plan, but I did a sim swap when I first got the phone. The process went like this.
Get iPhone 3G.
Call AT & T 2 days later saying that I lost my iPhone, would like to switch the data plan to a regular phones medianet for $15 a month. They said sure, and I needed to give them a regular (non-smart) phone IMEI. I gave them some LG flip phone's IMEI, and everything was fine. They still had me listed as an iPhone 3G so I kept getting an email once a month saying my data plan was wrong, but I checked all the data usages (once even 22mb downloaded at once from Cydia) and it said, 22mb, $0.00 on the bill. Paying $15 a month, happy as a clam.
They don't really audit phones, so it's not a real problem.